1. Field of the Invention
The present invention relates to an engine intake system and more particularly to an intake system for a multiple cylinder engine. More specifically, the present invention pertains to an engine intake system which utilizes an intake inertia effect and an intake resonance between engine cylinders.
2. Description of the Prior Art
Conventionally, it has been known to obtain an increased intake gas charge through utilizations of the intake inertia and the intake resonance effects. By the term utilization of "inertia effect", it is meant that the pressure wave produced in an intake passage during cyclical operations of the intake valve is effectively utilized to increase the intake gas charge by for example determining the valve timing so that a positive pressure arrives at the intake port at a final period of the intake stroke. In contrast to the inertia effect, the resonance effect is a phenomenon inherent to a multiple cylinder engine. In an engine wherein the resonance effect is utilized, the engine cylinders are divided into groups, each group consisting of cylinders which are not adjacent to each other in terms of the order of ignition, and the intake ports of the cylinders in the same group are communicated together through an intake passage so that the pressure waves produced at the intake ports are resonated in the intake passage. Thus, the pressure in the intake passage is significantly amplified and a supercharging effect is obtained. In order to obtain an increased supercharging effect, it is therefore necessary to have the natural frequency of the intake gas in the intake system appropriately matched with the frequency of the operation of the intake valve.
In an engine intake system wherein the cylinders are divided into a plurality of groups as described above, it is required for obtaining a satisfactory resonance effect to open the intake passages for the respective cylinder groups separately to the atmosphere because otherwise the intake pressure in one passage will interfere with the intake pressure in the other passage significantly weakening the pressure in the intake passages. In a conventional arrangement, the intake passage for one group is connected at the upstream end with the upstream end of the intake passage for the other group so that the pressures in the both passages are equilibrated at the upstream ends to thereby obtain an effect of opening the passages separately to the atmosphere.
It should however be noted that in the intake system wherein the upper ends of the intake passages for the respective cylinder groups are connected together as mentioned above, pressure transmittal in the intake passages for the same cylinder group is to some extent disturbed so that the pressure resonance effect is adversely decreased.
In Japanese utility model application No. 57-71445 filed on May 18, 1982 and published for public inspection on Nov. 22, 1983 under the disclosure No. 58-175121, there is disclosed an engine intake system including expansion chambers which are separated by a partition wall, each of the expansion chambers being connected with individual combustion chambers which are not adjacent to each other in terms of order of combustion through independent branch passages. The expansion chambers are respectively connected with resonance passages which are communicated with each other at the upstream ends. There is also provided an auxiliary resonance passage which has a length and a diameter substantially equal to those of the first mentioned resonance passages. In the proposed intake system, the pressure wave transmittal is still partly made through the main resonance passages through which the intake air flows. Therefore, the pressure wave transmittal is still to some extent disturbed and the resonance effect is decreased.
It is known that the natural frequency of the intake system is dependent on the length of the intake passage and the volume of the expansion chamber. Contrary to this, the frequency of the intake valve operation is determined by the engine speed. It will therefore be understood that it is required to change the length of the intake passage and the volume of the expansion chamber in accordance with the engine speed in order to obtain an effective supercharging function under the intake pressure resonance irrespective of the engine speed. In this view, Japanese patent application No. 56-33990 filed on Mar. 11, 1981 and published for public inspection on Sept. 13, 1982 under the disclosure No. 57-148025 proposes an intake system having an expansion chamber for each cylinder group and an intake passage extending from the expansion chamber toward upstream side, a passage length changing arrangement being provided in the vicinity of the junction between the intake passages for the respective cylinder groups. It should however be noted that the arrangement is disadvantageous in that a satisfactory resonance effect cannot be obtained since the passage for the intake air flow is used in common for the pressure wave transmittal.